1. Field of the Invention
The present invention relates to an extruded, smooth finished solid sheet or film, comprised of thermoplastic material, particularly polycarbonate plastic, distinguished by a fine thickness tolerance, low surface roughness, low curvature, low thermal shrinkage, and, preferably, low birefringence of light. All of these properties are important in the use of extruded sheet or film in the manufacture of optically readable data storage devices. The present invention also relates to a method of manufacturing such extruded solid sheet or film, in wide sizes, and use of said materials in the manufacture of optical data storage devices.
2. Discussion of the Background
The requirements applied to plastic sheet or film intended for use in the manufacture of optically readable data storage devices are generally known. See Hennig, J., 1986, "Polymere als Substrate fuer optische Plattenspeicher", Angew. Makromol. Chemie., vol. 145/146 pp. 391-409. According to Eur. Pat. B 8543, they should contain no foreign particles of size greater than 10 micron, and should not form any bubbles or cavities when thermoplastically processed; further, when formed they should display minimal optical birefringence.
According to Eur. Pat. A 199,824, molding compounds comprised of polymer component units having specific values of the difference of the positive and negative main polarizability are used, because then the orientations of the polymer molecules in the sheet or the like formed from such materials result in much less optical birefringence. Accordingly, optical data storage media with low optical birefringence can be formed from such plastics, with the size of the storage medium exceeding that of so-called "compact disks" (CDs) (120 mm). However, the range of choices for such plastics is very limited, and most available industrial plastics do not qualify.
Optical data storage media in the form of CDs are generally produced by injection molding. Other methods of manufacturing large-format optical data storage media begin with solid plastic sheets or films of thickness 0.1-2 mm which are subsequently provided with the information coding. Because the size of the characters is in the micron range and requires very accurate focusing of the reading system, the quality requirements applied to the sheet or film employed are very stringent. They involve, e.g., the surface roughness, the thickness tolerance, the curvature, and the thermal shrinkage. These quality requirements must be met uniformly over a large minimum extent--if possible, 300 mm or more in each direction over the surface.
Conventional extrusion technology is no longer capable of meeting these quality requirements under conditions where both the thickness of the sheet or film and the type of plastic must be freely selectable in response to demand. Various methods are known for improving the quality of extruded sheet or film, but these are inadequate to meet the requirements imposed.
Smooth finished extruded sheets of amorphous thermoplastic are conventionally produced by passing a slot-extruded strip through a polishing stack. In the gap between the rolls the surfaces of the strip are abruptly vitrified via the calendering rolls which are at a temperature below the glass transition temperature, so that only the interior of the strip remains in the thermoplastic state. In this process, an excess of the thermoplastic may be forced out of the roll gap against the direction of flow, forming a bead-like prominence on the strip in front of said gap.
If the strip is thinner than a certain thickness, the vitrified layers meet in the central plane, and therefore there is no longer any part of the thickness of the strip which is capable of flow; accordingly, the excess can no longer be forced out of the roll gap. As a result, the compression forces which develop in the roll gap are so great that the surfaces of the polishing rolls, or the bearings or stands of said rolls, may suffer damage. Such compression forces cannot be eliminated merely by reducing the extruding speed or increasing the turning speed of the calender, because then there is a hazard that the roll gap will not be fully occupied by the strip, resulting in failure to achieve the desired smoothing. It is impracticable to attempt to achieve the ideal state of a minimally thick bead having a uniform thickness over the entire width of the strip ahead of the roll gap, in the case of an intended thickness of the strip which is below a certain thickness. Smooth finished extruded sheets or films thinner than this cannot be produced by conventional calendering technology.
According to Ger. Pat. 2,432,778, thermoplastic films with advantageous surface properties, outstanding dimensional stability, and nearly isotropic properties may be produced by taking an extruded strip of thermoplastic material which has been extruded under minimal compression conditions and feeding it immediately after it emerges from the extruder to a take-up device comprised of two superposed synchronously driven endless conveyor belts between which the strip is compressed, widened, and pressed flat, wherewith the strip is carried along with and adheres to said belts and while disposed between said belts is cooled to below its glass transition temperature. At the entrance to the take-up device both conveyor belts are maintained at a temperature higher than the glass transition temperature of the plastic, and they are cooled in equal fashion in the later part of their excursion. The thickness tolerance achieved is 0.05-0.1 mm over a width of about 60 mm. It is not possible to produce optical birefringence with a difference in path of&lt;50 nm. Further, the tolerance value stated is inadequate for optical data storage media.
According to Jap. pat. App. 54/017,982 (1979), smooth finished extruded films comprised of hard thermoplastics with improved uniformity of thickness are obtained by driving the calender adjoining the extrusion nozzle with a drive system which prevents fluctuations in movement. Very accurate thermal control of the individual calendering rolls is necessary. This extrusion technique is also inadequate to overcome the problem of optical birefringence.
Jap. pat. App. 57/014,065 (1983) discloses an extrusion method wherein the extruded strip of thermoplastic is rested on an endless conveyor belt after exiting the nozzle, and is allowed to cool there. No means of smoothing the free upper surface of the strip are provided. The result is a strip with an underside which is flat and amenable to good adhesive bonding.
Ger. Pat. 3,429,818 describes a method of extruding films comprised of thermoplastic with low optical birefringence, whereby a core layer of the thermoplastic is coextruded with two outer layers of polyethylene or another plastic which does not adhere to the core layer. A single-layer, low-birefringence film is obtained from the three-layer co-extrudate, by pulling off the two outer layers from the core layer. This method does not enable high surface smoothness of the surfaces of the core layer.
Plastic sheet 1-2 mm thick has been offered commercially which does satisfy the quality requirements imposed on optical data storage media. However, experience has shown that the most advanced extrusion and smoothing techniques cannot be employed to produce such quality on a consistent and reproducible basis. Therefore there is a strong need for sheet and film material of the necessary quality, and for a method of producing same.
In particular, there remains a need for extruded solid plastic sheet and film for manufacturing optical data storage media which meet the following quality requirements:
1. An optical birefringence path difference not exceeding 50 nm in a single pass through the sheet or film;
2. Variation in thickness not exceeding 0.1 mm in the course of dimensions of up to 700.times.650 mm;
3. For sheet or film with thickness&lt;1.0 mm, variation in thickness&lt;0.05 mm in the course of dimensions up to 700.times.650 mm;
4. Variation in thickness not exceeding 0.04 mm, preferably not exceeding 0.025 mm, in the course of dimensions of up to 300.times.300 mm;
5. Maximum surface roughness (Rmax according to DIN 4768)&lt;300 nm on at least one surface, and/or mean surface roughness (Rz according to DIN 4768)&lt;150 nm, preferably &lt;100 nm;
6. Surface roughness at the center (Ra according to DIN 4768)&lt;40 nm, preferably&lt;25 nm;
7. Curvature&lt;1.6 mm in the course of dimensions of up to 300.times.300 mm (where curvature is defined as the maximum distance of the free-lying sheet or film from a plane support surface);
8. Shrinkage of not more than 16% in the extrusion direction and/or not more than 5% transversely to the extrusion direction, when stored 30 min at 160.degree. C.
While appropriate characteristics, configuration, and adjustment of the polishing stack will enable requirements 2-6 to be satisfied, for strip thicknesses greater than or equal to a critical value, there is no systematic way to approach the problem of meeting the other requirements, particularly those directed at achieving low birefringence. All previous attempts to achieve low birefringence have been limited by the constraint that they not interfere with or prevent the satisfaction of the other requirements.